The present invention relates to a planar resistance heating element composed of a resistive body having a stable positive temperature coefficient in a high-temperature range.
There are known electrically conductive polymers made of certain polymer materials having minute electrically conductive particles e.g. carbon, dispersed therein which exhibit a positive temperature coefficient (hereinafter referred to as a "PTC"). A conductive polymer with such a PTC is capable of limiting a current flowing therethrough, and hence is utilized as a temperature sensor or a heating element having an ability to control the temperature thereof.
As disclosed in U.S. Pat. Nos. 3,591,526 and 3,673,121, for example, a resistive body comprising a conductive polymer of the type described above is formed by mixing conductive particles such as of carbon black or graphite with a crystalline resin such as polyethylene or polypropylene, heating and kneading the mixture, and molding the mixture to a desired shape, or reducing the mixture to powder or fine pellets, mixing the powder or fine pellets with a molding resin, and molding the mixture to a desired configuration, thereby producing a resistance heating element.
The resistive body has a PTC characteristic since the crystalline resin changes from a crystalline structure to an amorphous structure to result in a sharp rise in resistance in the vicinity of a melting point thereof. The resistance of the crystalline resin becomes maximum in the vicinity of the melting point, but is abruptly reduced when the resistive body reaches a temperature beyond the melting point. If the temperature of the resistive body exceeds a certain value, it loses its ability to limit the current and the temperature thereof rises to the point where the resistive body will suffer from burning. The same problem occurs with a planar resistance heating element comprising a resistive body of a PTC patterned on a substrate as by screen printing. FIG. 2 of the accompanying drawings shows a temperature-resistance characteristic curve B indicating that when an ambient temperature T, given on a horizontal axis, is in excess of 110.degree. C., a resistance ratio R.sub.T /R.sub.20 is reduced, the resistance ratio being a ratio between a resistance R.sub.20 at a temperature of 20.degree. C. and a resistance R.sub.T at a certain temperature T.
Since the resistance of the resistive body is reduced at temperatures higher than the melting point of the crystalline resin, the resistive body has an insufficient ability to withstand heat with respect to the temperature. For example, if the resistive body is used in a resistance heating body with self temperature control, when the crystalline resin is heated to a temperature higher than the melting point for some reason, an excessive current flows throughout the resistive body which then may suffer burning. FIG. 3 shows a characteristic curve B of the conventional planar resistance heating element, described above, in an accelerated deterioration test in which a voltage of 500 V that is 5 times greater than the rated voltage is applied to the resistance heating element. The graph of FIG. 3 has a horizontal axis indicating the time (hour) during which the voltage is continuously applied and a vertical axis representative of a current (A). Study of the curve B clearly indicates that after the voltage has been applied for about 20 minutes, an excessive current flows and the resistive body is burned out.
Where the resistive body described above is employed as a resistance heating body with self temperature control, the resistive body will be soften in the vicinity of the controlled temperature and deformed if a slight external force is imposed on the body. It is also known that in the resistive body, the conductive particles generally have poor affinity with the crystalline resin, and the condition in which the conductive particles are dispersed in the crystalline resin is subjected to changes. Therefore, the resistance of the resistive body is unstable, and the PTC characteristics will be lost in a relatively short period of time.